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It's simple: because sheer PageRank is not the main ranking factor. For instance, it doesn't take into account if backlinks are related or not. Neither does it count all the ON-PAGE optimisation that may or may not have been done.

Google have penguin and panda algorithms to control search results and page rank i think your first site have higher rank cause after new upgrade of Google penguin back-linking is not so impotent .

Never mind the potency

The Penguin & Panda updates have not changed the fact that Page Rank was NEVER the most important factor in rankings.

Google themselves are constantly preaching that it is only one of over 200 factors used to rank your website.

Page Rank is affected by every (followed) backlink (relevant/related or not).
Rankings are greatly affected by backlinks from relevant highly trusted sites, but not so by backlinks from non-related non-trusted sites... (in fact since the Penguin update these can be harmful).
So... Page Rank is not - and never was - an indicator of a site's ranking, nor even an indicator of a 'sites potential ability to rank, it is simply indicative of the total backlink power aimed at the page... and in the world of SEO, it's main value was - and still is - for increasing value in sold links, (or selling the domain itself).